1. Field of the Invention
This invention generally relates to the packaging of microelectromechanical systems (MEMS) and, more particularly, to a system and method for simultaneously packaging a MEMS device with active circuitry on an integrated circuit (IC) substrate.
2. Description of the Related Art
MEMS devices are typically made on silicon wafers; using one of two well established techniques: bulk micro-machining or surface micro-machining. In both of these methods, the MEMS device is fabricated on a silicon wafer using standard IC-type fabrication equipment. Once the wafer is processed, the wafer is diced to form individual die. These MEMS die may or may not be integrated with electronic components (on CMOS). Once the die is cingulated, it must then be packaged in some form of package, similar to an IC package. This package is eventually inserted into a socket or bonded to a Printed Circuit Board (PCB) as part of an overall system, i.e., a cell phone. These packages can be quite elaborate, depending on the MEMS style and application, including vacuum package requirements. In addition, because many MEMS devices are required to move during operation, the package must provide a cavity that allows for this movement.
One problem with this type of MEMS packaging methodology is that the package is a very large proportion of the total MEMS device cost; on the order of 30–70% of the overall cost. This packaging cost can, therefore, have a significant impact on the capability of such MEMS devices to penetrate cost-sensitive markets, such as the cell phone market.
Another problem with existing MEMS packaging is the noise inherent with the electrical connections between the MEMS package and the rest of the system. The bonding, wiring, and electrical interconnections associated with interfacing a MEMS device embedded in a package, to a circuit, necessarily adds impedance mismatches that result in noisy or low amplitude signals.
However, there is mounting evidence that MEMS technology can add value to systems, such as cell phones, in a market that is ripe for new technology, if only the packaging issue could be addressed. Continuing with the cell phone example, it is certain that the camera-on-cell phone has made a great impact on the market. The search is on for the next added functionality that can drive new expansion of the cell phone market.
MEMS are being considered for the following cell phone functions:
1) Motion capture (Accelerometer and gyroscope);
2) Microphones;
3) RF devices and RF modules;
4) Image capture;
5) Low power solutions;
6) Identification (biometrics);
7) Enhanced display functionality; and,
8) Personal health and safety monitoring.
The issues preventing MEMS penetration into the cell phone market are cost and performance. As mentioned above, packaging is 30–70% of the MEMS device cost. This cost issue is preventing the integration of MEMS into cell phones, display systems, and many other types of electronic devices.
MEMS devices are a logical derivative of semiconductor IC processes that may be used to develop micrometer scale structural devices such as transducers or actuators, and they are typically fabricated on silicon substrates. MEMS devices typically interface physical variables and electronic signal circuits. The integration of MEMS into larger scale systems has been expensive to fabricate due to the process difficulties and the cost associated with integrating the MEMS standard IC technologies, such as CMOS. The processes used to fabricate MEMS on glass offer the advantage that the integration of electrical and mechanical functions is easily done. In this way, system level integration is possible and cost effective.
It would be advantageous if MEMS devices could be packaged as part of the overall process of fabricating active devices on a circuit board or display.